1. Field of the Invention
The present invention relates to a modified product mask for bridging detection on a semiconductor wafer.
2. Description of the Related Art
Metal or polysilicon (poly) bridging structures typically are small structures in the scribe line of a product mask. These bridging structures are used to determine if a semiconductor device formed on a wafer by a product mask is defective in the sense that bridging occurs between elements on the wafer that are supposed to be electrically separated from each other. The product mask is used to form a particular pattern onto a wafer, and is used along with a resist, such as a photoresist, to create the particular pattern. Etching is also performed to remove the resist and undesired portions of layers (e.g., poly, oxide, nitride) formed on the wafer.
A scribe line of a semiconductor wafer is the area separating individual chips on the wafer. After electrically testing the wafer to ensure that it operates properly, the wafer is partitioned into separate chips along the scribe line. A variety of testing structures are typically placed in the scribe line and are tested before the wafer is partitioned, since, after partitioning, the testing structures are removed from the actual product to be sold due to the separation of the chips from the testing structures on the scribe line.
The testing structures are typically very small since the scribe line is made as small as possible (in order to obtain the maximum possible useful area on a wafer). The small size of the testing structures is also due to the fact that there are many testing structures located on the scribe line, and these testing structures have to compete for a very small real estate on the wafer. Typically, the testing structures are made in accordance with a minimum-spacing/size design rule for the wafer. Due to their limited size and due to the testing area being much smaller than the actual product circuit, the testing structures are typically less sensitive to bridging than the actual circuit (the non-testing-structure part of the product mask).
Certain testing structures, called bridging structures, can be formed in various types of configurations. Bridging structures are testing structures that are specifically designed to test for bridging between elements or regions on a wafer. One such bridging structure 100, as shown in FIG. 1, includes conductive (e.g., metal) fingers 110A-110E that are electrically connected to each other via conductive line 115, conductive (e.g., metal) fingers 120A-120D that are electrically connected to each other via conductive line 125, and a serpentine conductive line 135 that snakes its way around each of the metal fingers 110A-110E, 120A-120D. The bridging structure 100 also includes first and second pads 145, 155 electrically connected to the conductive line 115, third and fourth pads 165, 175 electrically connected to the conductive line 125, and fifth and sixth pads 185, 195 electrically connected to the serpentine conductive line 135.
By providing a current to one pad, such as the fifth pad 185, it can be determined if that current flows to any of the first through fourth pads. If the product mask has been correctly constructed, then there should be no current flow, or bridging, between the conductive fingers 110A-110E, 120A-120D and the serpentine conductive line 135. If such a current flow is detected at any of the first through fourth pads, however, then the product mask fails the bridging test, and that wafer is determined to be defective.
The steps used to create a wafer with a testing region formed in the scribe line of a product mask are shown in FIG. 2, and described below.
In a first step 200, which may involve several process steps, transistor regions are formed, along with front ends for those devices. Poly deposition is also performed in this step. Then, in a second step 210, there is performed oxide deposition, polishing, and then contact formation. In a third step 220, a metal or poly deposition step is performed. In a fourth step 230, a photolithography step is performed. The photolithography step 230 includes a first substep of coating the wafer with a resist, a second substep of exposing the is resist with a product mask, and then a third substep of developing the pattern such as by dipping the wafer into a developing solution. In a fifth step 240, a metal or poly etching is performed in accordance with the developed pattern on the wafer. In a sixth step 250, an electrical bridging test is performed on the small scribe line structure of the wafer. In a seventh step 260, a yield test is performed, which is sensitive to the bridging in the circuit.
As mentioned above, the product mask with a small testing region formed in the scribe line has problems in that the testing region is typically less sensitive to bridging than the actual product region that it is meant to test the correct formation thereof.
Another way to test for bridging on wafers is to use a Defect Monitor Mask, or DMM, instead of the actual product mask. As opposed to the problems associated with the use of bridging structures formed only in the scribe line, the DMM forms bridging structures throughout the wafer, not just in the scribe line. The DMM is used instead of the product mask, and is used to form a special structure with many thousands of bridging structures formed throughout the wafer. Typically, the DMM is used for only a few wafers in a batch, in order to determine if the entire batch of wafers is being properly manufactured.
The process steps for forming a wafer according to the DMM are shown in FIG. 3 and described below.
In a first step 300, oxide is deposited or grown onto a silicon wafer. In a second step 310, metal or poly is deposited onto the wafer. In a third step 320, a photolithography process is performed. The third step includes the following substeps: a) coating the wafer with resist, b) exposing the wafer using a defect monitor pattern, and c) developing the pattern. In a fourth step 330, a metal or poly etching is performed in accordance with the developed pattern. In a fifth step 340, an electrical bridging test is performed for all of the bridging structures formed by way of the DMM.
A wafer formed with the DMM is very sensitive to bridging, and is actually too sensitive with respect to the actual product mask that it is supposed to represent in a testing phase. That is, the wafer formed with the DMM contains typically more metal than a wafer formed with a product mask, due to the larger number of testing structures formed by way of the DMM. Thus, with the increased metal density of the DMM-formed wafer, it can behave differently through processing steps of interest, such as patterning, cleaning, etc., with respect to a product-mask-formed wafer.
The DMM flow is short and sensitive, but the DMM pattern is quite different from the product mask, and thus the lithography and etching of the wafer with the DMM pattern may be different than a wafer formed with the product mask, due to a loading difference between the patterns. The DMM pattern typically has more metal left on the wafer after etch than the product mask, and this loading difference can lead to differences at lithography and at etch. These differences may result in bridging problems existing on the DMM-patterned wafer which do not occur in the product mask-patterned wafer, and vice versa.
Thus, it is desired to have a testing wafer that is both sensitive to bridging to the same extent as a wafer formed by way of a product mask, as well as having similar loading characteristics as the wafer formed by way of the product mask.